Position-measuring systems play an ever more important role in this increasingly automated world. They furnish the basis for exact positioning of drive systems in many applications, for example, in the field of machine tools. The optical position-measuring systems described herein are based on scanning a scale that has a measuring standard in the form of a line grating. The scanning head used for this includes a light source from which light falls on the scale grating via a transmitting grating. After the interaction with the transmitting grating and the scale grating, the light has a spatial intensity pattern which is able to be detected in the scanning head using a receiving grating and is able to be used for position determination.
It is conventional to form a photodetector from a plurality of photosensitive areas. These photosensitive areas are arranged in the scanning head such that they are able to record the different phases of the intensity pattern and to supply corresponding electrical output signals. The individual, evenly spaced photosensitive areas form a receiving grating, in this context.
Four signals may be generated that are offset by 90 degrees with respect to each other in each case, from which, in a sequential electronic system, counting signals connected with direction may be derived. In response to the shifting of the scale relative to the scanning head, the individual phase-shifted signals change as a function of position.
Usually, from the four output signals mentioned, first of all two signals shifted by 90 degrees with respect to each other and free from offset errors, amplitude errors and phase errors are synthesized, which are suitable for a finer subdivision and interpolation. The counting signals connected with direction are able to permit therewith a substantially finer position determination than would be possible, for example, by counting the maxima and/or minima of the intensity pattern at the photosensitive areas of the scanning head.
For reasons described further on, it may be provided that the individual photosensitive areas are as near as possible to one another. The use of discrete component parts, such as photodiodes, limits the possible miniaturization of the photodetectors. Therefore, structured photodetectors have been implemented which, using conventional process steps of microelectronics, permit the production of structured, photosensitive areas on one single semiconductor substrate.
Because of the low inclination to cross feed between the individual photosensitive areas, in this context, there is available, above all, the technologically well manageable amorphous silicon (a-Si), whose use for converting light to electric current is conventional, for example, in the solar cell field.
German Published Patent Application No. 101 29 334 describes an optical position-measuring system having a light-receiving device based on the principle described above. The photosensitive areas for scanning of locally intensity-modulated light of different phase positions are constructed as receiving gratings in the form of several semiconductor layer stacks of doped and undoped amorphous silicon. The construction of the structured detectors is very complex, however, so that the method for its production is also costly.